Pyrolyzed egg yolk as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions

Abstract

A novel electrocatalyst of heteroatom-doped carbon (HDC) has been developed via facile pyrolysis of hen egg yolk without incorporating external heteroatoms. This HDC showed a high electrocatalytic activity towards oxygen reduction/evolution reactions in alkaline media, which indicates that it might be a very promising alternative to costly Pt-based electrocatalysts.

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With increasing concerns over shortages and emissions of energy from fossil fuels and obtaining them, there have been intense efforts worldwide to develop sustainable and green technologies for energy conversion and storage. To this end, the development of efficient and affordable electrocatalysts towards the oxygen reduction reaction (ORR) and/or the oxygen evolution reaction (OER), such as those used in metal–air batteries, fuel cells and water splitting, is highly desirable.^1–3 While the materials based on Pt group metals remain the best available electrocatalysts for the ORR and/or the OER to date, the high cost and scarcity of these metals hinder their wide and large scale application.⁴ Therefore, low-cost yet efficient electrocatalysts consisting of earth-abundant elements need to be identified urgently.^4–6

Heteroatom-doped carbon (HDC) materials have emerged as promising metal-free electrocatalysts in recent years.^7,8 Heteroatoms modify the chemical and electronic properties of carbon structures and render substantial active sites for catalyzing the ORR or OER. Mono-doping and co-doping^9,10 of N,^11–13 S,¹³ P,¹⁴ B¹⁵ and F¹⁶ heteroatoms in the graphitic framework of carbon nanostructures were reported. The preparation of HDC materials was carried out using a wide variety of precursors, from inorganic and organic chemicals,^17,18 naturally occurring substances to biomasses,^19–22 by means of different methods, including pyrolysis,²³ hydrothermal/solvothermal synthesis,²⁴ chemical vapor deposition,²⁵ and plasma synthesis.²⁶ For most HDC electrocatalysts, the catalytic activity for either the ORR or the OER was studied widely. However, HDC electrocatalysts that were efficient towards both the ORR and the OER were rarely reported.^27,28

In this communication, we report on a novel electrocatalyst of HDC prepared via a one-step pyrolyzing process, using egg yolk as the precursor of graphitic carbon and the source of heteroatoms N, S and P. Egg yolk is known for being cheapen inexpensive and abundant source of high quality protein, but having a high content of cholesterol. Turning egg yolk into high performance HDC materials would be a new way to make use of it. The pyrolysis of yolk was carried out at different temperatures and the resultant HDC materials were tested and compared with a common commercial Pt/C electrocatalyst with respect to ORR and OER performance.

The synthesis scheme is illustrated in Fig. 1(a). The boiled egg yolk separated from hens' eggs was ground together with five times its weight of KCl and then heated in a tube furnace under constant Ar flow at a ramp rate of 10 °C min⁻¹ until a set temperature was reached. The yolk was pyrolyzed at the set temperature for 2 h to form HDC, which was then cooled and filtrated with water until no Cl⁻ could be detected in the filtrate. Finally, the filter cake of pyrolyzed yolk was dried at 80 °C and stored for later testing. Because the carbonaceous materials that embed N heteroatoms can be regarded as “carbon nitrides (CN)” materials,^29,30 the yolks pyrolyzed here at 700, 800, 900, and 1000 °C are designated as yolk-CN-700, yolk-CN-800, yolk-CN-900, and yolk-CN-1000, respectively. The effect of temperature on the structural and functional properties of the as-obtained HDC is studied via scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cyclic voltammetry (CV) and Linear Sweep Voltammetry (LSV), and the details of these characterizations are given in the ESI.†

Fig. 1 (a) Schematic of the synthesis process of the yolk-derived HDC catalysts. (b) SEM and (c) TEM images of the sample yolk-CN-800. (d) EDS spectra of the yolk-derived HDC catalysts at different temperatures.

Fig. 1(b and c) and S1 (shown in ESI†) show the microscopic images of yolk-CN-800, which are typical of HDC obtained at different temperatures. A layered structure consisting of loosely stacked graphitic nanosheets can be observed. The formation of this partly-structured carbon is presumably facilitated by molten KCl, in which yolk is dispersed in the process of carbonization. A similar molten salt effect on the carbonization of glucose was previously reported and discussed.²³

Besides the dominant element carbon, the yolk-derived HDC also contains the elements of oxygen, hydrogen, nitrogen, sulfur and phosphorus, as jointly detected by EDS (Fig. 1(d) and S2†) and XPS (Fig. 2(a), S4† and Table 1). A general trend of a decrease in nitrogen content with increased temperature of pyrolysis can be noted, because C–C and N≡N bonds form more favorably than C–N bonds at high temperatures.³¹ In contrast, the relative content of sulfur and phosphorus increases with pyrolyzing temperature (Table 1). This is different from the case of pyrolyzing keratin, in which both N and S contents drop with increased temperature.³² Presumably, the difference is due to the distinct disulfide and methylthio groups, to which sulfur in keratin and yolk, respectively, belong. Nitrogen in both the proteins comes from amide groups, leading to the similar content and variation of nitrogen with pyrolyzing temperature.

Fig. 2 XPS survey spectra of the HDC catalysts (a); XPS N 1s spectrum (b) and C 1s spectrum (c) of yolk-CN-800; XRD patterns of the HDC catalysts (d); Raman spectra of the HDC catalysts (e); deconvoluted Raman spectrum of yolk-CN-800 (f).

Table 1 Elemental contents of HDC catalysts from XPS measurements (at%)

HDC catalysts	N	N1^a	N2^a	N3^a	N4^a	S	P	N + S + P
a N1, N2, N3 and N4 indicate the percentages of pyridinic-N, pyrrolic-N, graphitic-N and oxidized-N, respectively, in the total N-containing species.
Yolk-CN-700	5.13	32.3	16.2	48.3	3.19	0.10	0.31	5.54
Yolk-CN-800	5.30	34.1	11.3	49.7	4.95	0.13	0.39	5.82
Yolk-CN-900	0.85	27.2	18.2	45.9	8.75	0.14	0.34	1.33
Yolk-CN-1000	1.48	19.8	7.26	67.8	5.21	0.16	0.77	2.41

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It is known that N-containing species in a HDC material are key contributors to its electrocatalytic activity towards the ORR or OER.^33,34 In particular, pyrrolic-N³³ and pyridinic-N³⁴ are identified as being very electrocatalytically active. The deconvoluted XPS spectra of C 1s and N 1s (Fig. 2(b and c)) distinguish four different N-containing species in the yolk-derived HDC (Table 1). Among the four HDC catalysts, yolk-CN-800 has the highest fraction of pyridinic-N, as well as the highest total content of doped nitrogen.

A turbostratic carbon structure of the yolk-derived HDC is indicated by XRD patterns shown in Fig. 2(d), which are characterized by two broad peaks centered at ca. 24° and 44°, corresponding to the (002) and (101) lattice planes of the carbon structure. The peaks corresponding to the (002) plane slightly shift to higher angles and have higher intensity with the increase in pyrolyzing temperature, as high temperature would lead to a more graphite-like structure in the HDC. In addition, the peaks of the (002) plane are on the left side of the graphite peak (26.5°), implying that the interlayer spacing between the stacks of thin carbon sheets is larger in the HDC than in graphite.

The Raman spectra of yolk-derived HDC (Fig. 2(e and f) and S3†) indicate a structural disorder introduced by heteroatom doping, as well as incomplete graphitization. A deconvolution of the spectra reveals four peaks for each of the HDCs, i.e. the peaks shown in Fig. 2(f). The D and G bands at ca. 1360 and 1600 cm⁻¹ are characteristic of a graphitic structure,³⁵ whereas the peaks at 1160 and 1520 cm⁻¹ correspond to the ν3 and ν1 modes of trans-polyacetylene.³⁶ The ratio of D to G band intensity I_D/I_G, a measure of the divergence from graphitization,³⁵ decreases as the of pyrolyzing temperature increases (Fig. 2(e)). However, even for yolk-CN-1000, the degree of graphitization is still as low as 0.29. The relatively broader and higher D band, corresponding to yolk pyrolyzed at a lower temperature, is associated with a higher concentration of doped heteroatoms, which results in defects in the graphitic structure.³⁷

The performance of the yolk-derived HDC catalysts towards the ORR was tested via CV and LSV. From the CV and polarization curves shown in Fig. 3(a and b), it is observed that yolk-CN-800 has the most positive peak potential and onset potential, respectively, indicating higher ORR activity as compared with the other three HDC catalysts. Moreover, yolk-CN-800 is comparable to the commonly used commercial Pt/C, with an onset potential shifted by only −49 mV, but a higher ORR current at high overpotentials (Fig. 3(b)). The ORR performance of the yolk-derived HDC catalysts seems to be associated with the content of heteroatom-containing species they possess, e.g. the best performing yolk-CN-800 catalyst contains the highest content of pyridinic nitrogen, total nitrogen and total heteroatoms. However, the relationships between each heteroatom-containing species and ORR performances are yet to be determined.

Fig. 3 (a) CVs of the HDC catalysts in N₂-saturated (dashed line) and O₂-saturated (solid line) 0.1 M KOH. (b) Polarization curves for the ORR in O₂-saturated 0.1 M KOH at a scan rate of 10 mV s⁻¹ with a rotating speed of 1600 rpm. (c) LSVs at different rotating speeds for yolk-CN-800. (d) The number of electrons transferred as a function of potential for different HDC catalysts. (e) Polarization curves for the OER in O₂-saturated 0.1 M KOH solution at a scan rate of 10 mV s⁻¹ with a rotating speed of 1600 rpm. (f) Durability of yolk-CN-800 and JM-Pt/C for the OER.

The polarization curves of yolk-CN-800 measured at different rotating speeds is shown in Fig. 3(c). The data obtained from such curves can be used to calculate the number of electrons transferred for each reduced oxygen molecule by means of the Koutecky–Levich equation³⁸ and the calculated results are given in Fig. 3(d). For yolk-CN-1000, the number of electrons transferred at low overpotentials is close to 2, meaning that the ORR is dominated by a two-electron process forming HO₂⁻,³⁹ whereas the number approaches 4 at high overpotentials and a OH⁻ forming process prevails. However, yolk-CN-800 is like a Pt/C catalyst in that the number of electrons transferred is approximately 4 over the whole range of potentials.

The performance of the yolk-derived HDC catalysts towards the OER was demonstrated via LSV. Again, yolk-CN-800 outperforms the Pt/C catalyst, as well as the other three HDC catalysts, on account of its low onset potential and high current density. Yolk-CN-800 shows an overpotential of 506 mV, which is 149 mV lower than the overpotential of Pt/C at the same current density (Fig. 3(e)) at 10 mA cm⁻². The stability of yolk-CN-800 over a Pt/C catalyst under OER conditions was shown via chronoamperometry at 702 mV and 851 mV, which correspond to the same current density of 10 mA cm⁻² in the respective polarization curve (Fig. 3(e)). Yolk-CN-800 retains 56% of its current density after more than 10 000 seconds of testing, while Pt/C retains 52% (Fig. 3(f)). Therefore, yolk-CN-800 is superior to Pt/C in terms of OER activity and stability. Moreover, the yolk-derived HDC catalyst, yolk-CN-800, appears to be the best one among the various metal-free HDC catalysts reported.⁴⁰

Conclusions

An efficient electrocatalyst of heteroatom-doped carbon (HDC) for both the ORR and the OER can be facilely prepared by pyrolyzing hens' egg yolk, taking advantage of the endogenous heteroatoms in the natural protein. The yolk-derived HDC catalyst shows a layered structure consisting of loosely-stacked graphitic nanosheets and demonstrates similar ORR activity and higher OER activity and stability in alkaline media compared with a commonly used commercial Pt/C catalyst. Therefore, the HDC in this report emerges as a very promising alternative to costly Pt-based electrocatalysts and is expected to find application in metal–air batteries, regenerative fuel cells and electrochemical oxygen pumps.

Acknowledgements

We acknowledge the financial support from the Natural Science Foundation of China (21120102039) and the Program of Introducing Talents of Discipline to Universities (B06006).

Notes and references

1. Y. Yan, J. Miao, Z. Yang, F.-X. Xiao, H. B. Yang, B. Liu and Y. Yang, Chem. Soc. Rev., 2015, 44, 3295–3346.
2. Z. Ma, X. Yuan, L. Li, Z.-F. Ma, D. P. Wilkinson, L. Zhang and J. Zhang, Energy Environ. Sci., 2015, 8, 2144–2198.
3. F. Cheng and J. Chen, Chem. Soc. Rev., 2012, 41, 2172–2192.
4. Y. Nie, L. Li and Z. Wei, Chem. Soc. Rev., 2015, 44, 2168–2201.
5. D. Geng, N. Ding, T. S. Andy Hor, Z. Liu, X. Sun and Y. Zong, J. Mater. Chem. A, 2015, 3, 1795–1810.
6. M. Sun, H. Liu, Y. Liu, J. Qu and J. Li, Nanoscale, 2015, 7, 1250–1269.
7. D.-W. Wang and D. Su, Energy Environ. Sci., 2014, 7, 576–591.
8. X. Wang, G. Sun, P. Routh, D.-H. Kim, W. Huang and P. Chen, Chem. Soc. Rev., 2014, 43, 7067–7098.
9. D.-w. Kim, O. L. Li and N. Saito, Phys. Chem. Chem. Phys., 2015, 17, 407–413.
10. D. C. Higgins, M. A. Hoque, F. Hassan, J.-Y. Choi, B. Kim and Z. Chen, ACS Catal., 2014, 4, 2734–2740.
11. T. Yang, J. Liu, R. Zhou, Z. Chen, H. Xu, S. Z. Qiao and M. J. Monteiro, J. Mater. Chem. A, 2014, 2, 18139–18146.
12. N. Brun, S. A. Wohlgemuth, P. Osiceanu and M. M. Titirici, Green Chem., 2013, 15, 2514–2524.
13. J. P. Paraknowitsch and A. Thomas, Energy Environ. Sci., 2013, 6, 2839–2855.
14. C. H. Choi, S. H. Park and S. I. Woo, J. Mater. Chem., 2012, 22, 12107–12115.
15. Y. Zhou, C. H. Yen, S. Fu, G. Yang, C. Zhu, D. Du, P. C. Wo, X. Cheng, J. Yang, C. M. Wai and Y. Lin, Green Chem., 2015, 17, 3552–3560.
16. X. Sun, Y. Zhang, P. Song, J. Pan, L. Zhuang, W. Xu and W. Xing, ACS Catal., 2013, 3, 1726–1729.
17. D.-S. Yang, D. Bhattacharjya, S. Inamdar, J. Park and J.-S. Yu, J. Am. Chem. Soc., 2012, 134, 16127–16130.
18. Y. Zhao, R. Nakamura, K. Kamiya, S. Nakanishi and K. Hashimoto, Nat. Commun., 2013, 4.
19. C. H. Choi, S. H. Park and S. I. Woo, Green Chem., 2011, 13, 406–412.
20. H. Xiong, H. N. Pham and A. K. Datye, Green Chem., 2014, 16, 4627–4643.
21. Z. Y. Wu, C. Li, H. W. Liang, J. F. Chen and S. H. Yu, Angew. Chem., Int. Ed., 2013, 52, 2925–2929.
22. H. Zhu, J. Yin, X. L. Wang, H. Y. Wang and X. R. Yang, Adv. Funct. Mater., 2013, 23, 1305–1312.
23. X. Liu, C. Giordano and M. Antonietti, Small, 2014, 10, 193–200.
24. J. Liang, Y. Jiao, M. Jaroniec and S. Z. Qiao, Angew. Chem., Int. Ed., 2012, 51, 11496–11500.
25. D. Yu, Y. Xue and L. Dai, J. Phys. Chem. Lett., 2012, 3, 2863–2870.
26. J. Senthilnathan, K. S. Rao and M. Yoshimura, J. Mater. Chem. A, 2014, 2, 3332–3337.
27. Y. Cheng, Y. Tian, X. Fan, J. Liu and C. Yan, Electrochim. Acta, 2014, 143, 291–296.
28. L. Wang, F. Yin and C. Yao, Int. J. Hydrogen Energy, 2014, 39, 15913–15919.
29. E. Negro, K. Vezzù, F. Bertasi, P. Schiavuta, L. Toniolo, S. Polizzi and V. Di Noto, ChemElectroChem, 2014, 1, 1359–1369.
30. V. Di Noto, E. Negro, S. Polizzi, F. Agresti and G. A. Giffin, ChemSusChem, 2012, 5, 2451–2459.
31. Z. Mo, S. Liao, Y. Zheng and Z. Fu, Carbon, 2012, 50, 2620–2627.
32. K. N. Chaudhari, M. Y. Song and J. S. Yu, Small, 2014, 10, 2625–2636.
33. S. M. Unni, S. Devulapally, N. Karjule and S. Kurungot, J. Mater. Chem., 2012, 22, 23506–23513.
34. C. KokáPoh, Energy Environ. Sci., 2012, 5, 7936–7942.
35. P. K. Chu and L. Li, Mater. Chem. Phys., 2006, 96, 253–277.
36. A. Ferrari and J. Robertson, Phys. Rev. B: Condens. Matter Mater. Phys., 2001, 63, 121405.
37. E. N. Nxumalo and N. J. Coville, Materials, 2010, 3, 2141–2171.
38. A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, Wiley New York, 1980.
39. M. Lefèvre and J.-P. Dodelet, Electrochim. Acta, 2003, 48, 2749–2760.
40. Z. Lin, G. H. Waller, Y. Liu, M. Liu and C.-p. Wong, Carbon, 2013, 53, 130–136.

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Footnotes

† Electronic supplementary information (ESI) available: The details of characterization (SEM, XPS, XRD, CV and LSV), HR-TEM images, EDS, Raman and N 1s spectra of the catalysts. See DOI: 10.1039/c5ra22066a

‡ These authors contributed equally.

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